Two explosions near the finish line of the Boston Marathon on Monday killed three people and wounded scores. A look at the facts in the case:

___

THE EXPLOSIONS

Two bombs exploded about 10 seconds and 100 yards apart at about 2:50 p.m. Monday in Boston’s Copley Square, near the finish line of the Boston Marathon. An 8-year-old boy, a 29-year-old woman and a Boston University graduate student from China were killed, and more than 170 were wounded. The explosions occurred about four hours into the race and two hours after the winners had crossed the finish line, but thousands of runners were still on the course.

___

THE INVESTIGATION

A department-store surveillance-camera image shows a man dropping off a bag at the scene of the one of the blasts, a top city politician who was briefed by police said Wednesday. He said officers are chasing leads that could take them to a suspect.

Authorities have said they believe the bombs were fashioned out of ordinary kitchen pressure cookers packed with explosives, nails, ball bearings and metal shards. They suspect the bombs were hidden in duffel bags and left on the ground.

___

THE VICTIMS

The 8-year-old boy killed in the bombings, Martin Richard, was remembered by friends and neighbors as a vivacious boy who loved to run, climb and play sports. Also killed was Krystle Campbell, a 29-year-old restaurant manager from Medford, Mass., whose father, William Campbell, said she had gone with a friend to watch the race.

The third victim, Lu Lingzi, was a graduate student at Boston University studying mathematics and statistics, according to the school. She was scheduled to receive her graduate degree in 2015.

Officials at three hospitals that treated some of the most seriously injured survivors said Wednesday they expect all of their patients to survive. At least 10 hospitals treated patients following the bombings, and at least 14 were still in critical condition Wednesday. Dozens of others have been released.

____

PRESIDENTIAL RESPONSE

President Barack Obama called the bombings an act of terrorism but said investigators do not know whether they were carried out by an international organization, a domestic group or a “malevolent individual.” He said, “The American people refuse to be terrorized.”

___

SECURITY RESPONSE

The area around Copley Square was closed after the attacks. On Wednesday, the federal courthouse in Boston was evacuated for about an hour because of a bomb threat.

___

WHAT’S NEXT

Law enforcement agencies have pleaded for the public to come forward with photos, videos or any information that might help them solve the case.